1. Field of the Invention
This invention relates to a video system with a self-diagnose function and, more particularly, to a test device arranged in a video signal processing apparatus and especially used in a television receiver or the like, using a video signal mixer for superposing character or graphic data on a video signal and displaying them on a screen of a display unit such as a CRT.
2. Description of the Related Art
In a recent video apparatus, a so-called on-screen function is added, which serves to accurately display an operation state of the apparatus on a screen using graphic or character patterns, thereby facilitating operation for a user. Typical examples of this type are reception channel display and a volume display during volume control in a color television receiver. Such a function can be realized by utilizing a character generator for generating graphic and character patterns to be displayed, and a video signal mixer for superposing character and graphic data on a video signal.
In general, the character generator is controlled in accordance with external control data using a microcomputer because the control requires various operations by means of external control data. A system including a microcomputer controls not only the character generator but also various functions. For example, in a satellite broadcast receiver, the microcomputer controls various functions related to a channel selection operation, an AFT operation, a video mute operation, an operation for switching the audio modes (including display), i.e., stereo and monaural modes, an audio mute operation, a key scanning operation for an input operation system, and the like in accordance with external control signals.
As the video system is allowed to increase its functions and receive a large quantity of external control signals and data by using the microcomputer in the above manner, the number of input and output ports (terminals) for the microcomputer is increased.
If the number of input/output terminals is increased, in order to increase utilization efficiency in each terminal, each terminal is time-divisionally used and the wiring for control signals becomes complicated. When any one of the wirings is disconnected or damaged due to a short circuit or the like, in a conventional test, each terminal is subjected to measurement by means of a simple measurement device generally called a tester such as a root-mean-square voltmeter or a root-mean-square ammeter. However, since the logical state of each terminal is changed with a lapse of time, it is very difficult to detect a defective portion.
If it is desirable to find a defective portion quickly, an expensive measuring device such as a logic analyzer is required.